The British Journal of Psychiatry (2008) 193: 185-191. doi: 10.1192/bjp.bp.108.051904
© 2008 The Royal College of Psychiatrists
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IQ and non-clinical psychotic symptoms in 12-year-olds: results from the ALSPAC birth cohort

Jeremy Horwood, BSc

Department of Social Medicine, University of Bristol, UK

Giovanni Salvi, MBChB

The Academic Unit of Psychiatry, University of Bristol, UK

Kate Thomas, BSc, MSc, Larisa Duffy, BSc and David Gunnell, PhD

Department of Social Medicine, University of Bristol, UK

Chris Hollis, PhD, DCH, MRCPsych

Division of Psychiatry, University of Nottingham, UK

Glyn Lewis, FRCpsych, PhD

The Academic Unit of Psychiatry, University of Bristol, UK

Paulo Menezes, MD, PhD

The Academic Unit of Psychiatry, University of Bristol, UK, and Department of Preventive Medicine, University of Sao Paulo, Brazil

Andrew Thompson, MA, MMedSci, MRCPsych

The Academic Unit of Psychiatry, University of Bristol, UK, and Orygen Research Centre, Department of Psychiatry, University of Melbourne, Australia

Dieter Wolke, DiplPsych, PhD

Department of Psychology, University of Warwick, UK

Stanley Zammit, PhD

The Academic Unit of Psychiatry, University of Bristol, UK, and Department of Psychological Medicine, Cardiff University, UK

Glynn Harrison, MD, FRCPsych

The Academic Unit of Psychiatry, University of Bristol, UK

Correspondence: Professor Glynn Harrison, The Academic Unit of Psychiatry, Community Based Medicine, Cotham House, Cotham Hill, Bristol BS6 6JL, UK Email: G.Harrison{at}bristol.ac.uk

Declaration of interest

None. Funding detailed in Acknowledgements.

Background

Non-clinical psychotic symptoms appear common in children, but it is possible that a proportion of reported symptoms result from misinterpretation. There is a well-established association between pre-morbid low IQ score and schizophrenia. Psychosis-like symptoms in children may also be a risk factor for psychotic disorder but their relationship with IQ is unclear.

Aims

To investigate the prevalence, nature and frequency of psychosis-like symptoms in 12-year-old children and study their relationship with IQ.

Method

Longitudinal study using the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort. A total of 6455 children completed screening questions for 12 psychotic symptoms followed by a semi-structured clinical assessment. IQ was assessed at 8 years of age using the Wechsler Intelligence Scale for Children (3rd UK edition).

Results

The 6-month period prevalence for one or more symptoms was 13.7% (95% CI 12.8–14.5). After adjustment for confounding variables, there was a non-linear association between IQ score and psychosis-like symptoms, such that only those with below average IQ score had an increased risk of reporting such symptoms.

Conclusions

Non-clinical psychotic symptoms occur in a significant proportion of 12-year-olds. Symptoms are associated with low IQ and also less strongly with a high IQ score. The pattern of association with IQ differs from that observed in schizophrenia.


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